The present invention relates generally to data communication devices, and specifically to adaptive digital data receivers.
Decision feedback equalizers (DFEs) are commonly used in digital data receivers. The DFE operates in conjunction with a decision device, such as a slicer, which estimates an output symbol value for each sample of a signal that it receives. The DFE provides feedback to the decision device based on prior decisions (i.e., symbols already estimated), with the purpose of canceling the effect of postcursor intersymbol interference in the current decision. For this purpose, the DFE typically comprises a multi-tap finite impulse response (FIR) filter, which receives as its input the sequence of symbols estimated by the decision device. The coefficients of the filter are generally determined adaptively, based on the characteristics of the channel through which the signal is received. The output of the DFE is then subtracted from the input of the decision device.
A comprehensive discussion of decision feedback equalizers is presented, for example, by Gitlin et al., in Data Communications Principles (Plenum Press, 1992), which is incorporated herein by reference. (See particularly pages 500-511.) Methods of filter adaptation are described, for example, by Haykin in Adaptive Filter Theory (3rd edition, Prentice Hall, 1996), which is also incorporated herein by reference.
It is also possible to use an infinite impulse response (IIR) filter in the DFE, as described, for example, by Crespo, in xe2x80x9cPole-Zero Decision Feedback Analyzer with a Rapidly Converging Adaptive IIR Algorithm,xe2x80x9d published in IEEE Journal on Selected Areas in Communications 9(6) (August, 1991), pages 817-828, which is incorporated herein by reference. The author describes a DFE containing a feedback filter composed of two sections: a relatively short FIR that cancels the initial part of the channel impulse response, and an IIR filter that cancels the smoothly decaying tail of the impulse response. This DFE offers a reduction in mean squared error relative to a conventional DFE having the same number of coefficients, with a comparable speed of convergence.
It is an object of the present invention to provide improved devices and methods for digital feedback equalization.
It is a further object of some aspects of the present invention to provide devices and methods for compensating for intersymbol interference effects that occur due to filtering of a signal transmitted over a channel, particularly due to notch filtering.
In preferred embodiments of the present invention, a digital filter with a known filter response is applied to a signal that is transmitted over a channel to a digital data receiver. Typically, the digital filter comprises a notch filter, which is preferably used in a transmitter to shape the signal that is transmitted to the receiver, or is alternatively used in the receiver to filter out narrowband interference. The receiver comprises a DFE having two parts: an adaptive part, which compensates for variable intersymbol interference effects, typically arising due to distortion in the channel; and a fixed part, which compensates for the intersymbol interference caused by the digital filter. Both the adaptive and fixed parts comprise multi-tap filters.
By separating the fixed and adaptive parts of the DFE, the length of the adaptive part of the DFE is reduced, relative to the number of taps that would be needed to achieve comparable performance in a conventional DFE with only a single (all adaptive) part. The DFE of the present invention thus converges more rapidly to the desired response than the conventional DFE would be able to do under these circumstances. Essentially, the fixed part of the DFE nullifies any system performance penalty that might otherwise result from the use of the digital filter, as long as the receiver also includes a feed-forward equalizer long enough to cancel signal precursors. Furthermore, regardless of whether the digital filter has a finite impulse response (FIR) or an infinite impulse response (IIR) structure, the DFE of the present invention is capable of compensating for its effect by using a FIR or IIR accordingly in the fixed part of the DFE.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a digital data receiver, for receiving a signal that is transmitted over a channel having a channel response and is filtered with a predetermined filter response, the receiver including:
a decision device, coupled to receive a sequence of equalized samples and to generate, responsive thereto, a corresponding sequence of estimated symbols;
a feed-forward equalizer, coupled to receive a sequence of input samples of the signal and to generate, responsive to the input samples, forward-equalized samples;
a decision feedback equalizer (DFE), coupled to receive the sequence of estimated symbols, and including:
an adaptive element, operative to process the estimated symbols responsive to the channel response; and
a fixed element, operative to process the estimated symbols responsive to the predetermined filter response, the DFE generating feedback-equalized samples by combined operation of the adaptive and fixed elements; and
an adder, coupled to combine the forward-equalized samples with the feedback-equalized samples so as to provide the equalized samples to the decision device.
Preferably, the receiver includes a filter, which is coupled to apply the predetermined filter response to the signal received from the channel. Alternatively, the predetermined filter response is applied by a transmitter, which transmits the signal over the channel.
In a preferred embodiment, the predetermined filter response includes a notch filter response. In another preferred embodiment, the predetermined filter response is applied by a digital filter having an infinite impulse response, and the fixed element of the DFE includes an infinite impulse response filter element.
Preferably, the adaptive element includes:
a decision feedback filter having adaptive coefficients; and
an adaptation circuit, operative to determine the adaptive coefficients responsive to the predetermined filter response, most preferably using a least-mean-square method and based on filtering the estimated symbols using the predetermined filter response.
Preferably, the predetermined filter response is described by a frequency response F(z), and the fixed element of the DFE includes:
a first fixed element, having a frequency response substantially equal to F(z), which operates on the estimated symbols in conjunction with the adaptive element to generate a first output; and
a second fixed element, having a frequency response substantially equal to (F(z)xe2x88x921)/zxe2x88x921, which operates on the estimated symbols to generate a second output,
wherein the DFE includes an adder, which sums the first and second outputs to generate the feedback-equalized samples.
In a preferred embodiment, the adaptive element includes a decision feedback filter having adaptive coefficients, which are determined responsive to the channel response in an initial stage of operation of receiving the signal from a transmitter, wherein in a subsequent stage of receiving the signal, the adaptive coefficients are conveyed to a precoder in the transmitter that is operative to precode the signal using the coefficients. Preferably, in the subsequent stage of receiving the signal, the adaptive element of the DFE is substantially inoperative, while the fixed element continues to process the estimated symbols responsive to the predetermined filter response.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for decision feedback equalization, including:
generating a sequence of estimated symbols based on a corresponding sequence of equalized samples, which are derived from a signal that is transmitted over a channel having a channel response and is filtered with a predetermined filter response;
processing the estimated symbols with adaptive processing parameters responsive to the channel response so as to generate adaptive feedback samples;
processing the estimated symbols with fixed processing parameters responsive to the predetermined filter response so as to generate fixed feedback samples; and
combining the adaptive feedback samples with the fixed feedback samples so as to generate feedback-equalized samples, for use in deriving the equalized samples.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: